The manufacture of acetic acid by carbonylating methanol in the presence of a catalyst is of major industrial importance as acetic acid is employed in a wide variety of applications. The reaction for producing acetic acid can be represented by the following equation:CH3OH+CO→CH3COOHHowever, the underlying chemistry is intricate and involves multiple interrelated reactions, by-products, and equilibria. To be practicable, a manufacturing process, therefore, has to balance those reactions, the associated by-products, and the purification of the product.
Prior to 1970, acetic acid was produced using a cobalt catalyst. A rhodium carbonyl iodide catalyst was developed in 1970 by Monsanto. The rhodium catalyst is considerably more active than the cobalt catalyst, which allows lower reaction pressure and temperature. Most importantly, the rhodium catalyst gives high selectivity to acetic acid.
One of the problems associated with the original Monsanto process is that a large amount of water (about 14% by weight of the reaction mixture) is needed to produce hydrogen in the reactor via the water-gas shift reactionCO+H2OCO2+H2 
Water and hydrogen are necessary to react with precipitated Rh(III) and inactive [Rh4(CO)2] to regenerate the active Rh(I) catalyst. However, a large amount of water increases the formation of hydrogen iodide which, in turn, increases the formation of undesired by-products, such as long chain alkyl iodides, which are hard to separate from the acetic acid product. Further, removing a large amount of water from the acetic acid product renders the process more costly.
In the late 1970s, Celanese modified the carbonylation process by introducing lithium iodide to the reaction mixture. Lithium iodide increases the catalyst stability by minimizing side reactions which produce inactive Rh(III) species. Consequently, the amount of water which is necessary to stabilize the catalyst can be reduced. Additionally, lithium iodide has been found to decrease the vaporization tendency of water. See, e.g., European Publication 506 240. The process, thus, has advantages with regard to the separation of water and acetic acid.
Additionally, it has been discovered that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 4%-wt. or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14 wt. % or 15 wt. % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium, at least a finite concentration of water, methyl acetate and methyl iodide, a specified concentration of iodide ions over and above the iodide content that is present as methyl iodide or other organic iodide. By using relatively high concentrations of the methyl acetate and iodide salt, a surprising degree of catalyst stability and reactor productivity has been achieved even when the water content of the liquid reaction medium is as low as about 0.1 wt. %. See, e.g., U.S. Pat. No. 5,001,259, U.S. Pat. No. 5,026,908 and U.S. Pat. No. 5,144,068. However, although the low water carbonylation process for the production of acetic acid reduces such by-products as carbon dioxide, hydrogen, and propionic acid, the amount of other impurities, present generally in trace amounts, is increased, and the quality of acetic acid sometimes suffers when attempts are made to increase the production rate by improving catalysts, or modifying reaction conditions.
Typically, acetic acid is produced in a plant which can be conveniently divided into three functional areas, i.e., the reaction, the light ends recovery, and the purification. In general, the reaction area comprises a reactor or reaction zone and a flash tank or flash zone. The light ends recovery area comprises a light ends distillation column or fractioning zone (also referred to in the art as “splitter” or “splitter column”) and a phase separation vessel, e.g., a decanter. The light ends distillation column may also be part of the purification area, which in turn further comprises a drying column and optionally a heavy ends distillation column. See, e.g., U.S. Pat. No. 6,552,221.
The light ends recovery area inter alia serves to separate undesired by-products such as alkanes, carbonyl impurities, and alkyl iodide impurities. The overhead stream which is recovered from the light ends distillation column is condensed and phase separated in the decanter to obtain a light, aqueous phase comprising primarily acetic acid and water, and a heavy, organic phase comprising primarily methyl iodide, methyl acetate, and alkane impurities. The aqueous phase which is obtained in this manner can be treated to remove acetaldehyde and other carbonyl impurities before being recycled, e.g., to the light ends distillation column. See, e.g., U.S. Pat. No. 5,599,970, U.S. Pat. No. 5,625,095, U.S. Pat. No. 5,732,660, U.S. Pat. No. 5,783,731, U.S. Pat. No. 6,143,930, European Publication No. 0 487 284. The organic phase can be further purified to remove, e.g., the alkane impurities, and at least part of the purified methyl iodide is returned to the process. See, e.g., U.S. Pat. No. 4,102,922, U.S. Pat. No. 5,371,286, U.S. Pat. No. 5,723,660, and U.S. Pat. No. 7,812,191.
The proper operation of the decanter is a critical part of the overall performance of the acetic acid process. The phase separation time must be shorter than the residence time of the mixture to be phase separated in the decanter in order to ensure sufficient recycle of the methyl iodide promoter to the reaction zone which, in turn, ensures that the reaction rate in the reaction zone is maintained. If the phase separation in the decanter is incomplete, the methyl iodide phase which is recovered from the decanter is diluted. Recycling of the diluted methyl iodide causes destabilization of the reactor conditions manifested by, e.g., (1) upset of the water balance in the reactor; (2) increased energy consumption; (3) decreased reaction rate; and/or, (4) increased catalyst consumption. Additionally, dilution of the methyl iodide phase alters its density which interferes with the operation of downstream pumps and other in-line equipment.
However, as the water concentration in the reaction mixture is lowered (also referred to as “low water-high acid” or “low-water” conditions) and the methyl acetate concentration increases, the vapor load of the light ends distillation column increases which, in turn, causes a high carry-over of acetic acid into the decanter. The solubility of acetic acid in both the methyl iodide and aqueous phases causes the phase separation to deteriorate, eventually resulting in a single liquid phase in the decanter. When this condition occurs, the aqueous stream which is returned from the decanter to the light ends column includes a high amount of methyl iodide as well as impurities. The presence of this additional methyl iodide and impurities further interferes with the ability of the light ends column to cleanly separate light ends materials such as methyl acetate and impurities from the acetic acid product. Additionally, the failure of the condensed light ends overhead to separate into two phases in the decanter under low water-high acid process conditions interferes with the removal of undesired by-products from the process.
The problem of efficient and thorough phase separation in the decanter under low-water process conditions is known in the art and attempts have been made to ensure proper phase separation of the condensed overhead stream in the decanter. For example, U.S. Pat. No. 5,723,660 proposes to reduce the amount of methyl acetate, to significantly reduce the temperature to which the light ends overhead is cooled before it enters the decanter, or to batch-wise feed water into the light ends column to ensure that the methyl acetate concentration remains below 40 weight percent. However, these measures increase the process steps, thus increasing the costs. Also, feeding water into the light ends column to ensure that the methyl acetate concentration remains below 40 weight percent, is likely to significantly alter the water balance throughout the process each time water is added. An alternative approach to improving the phase separation in the decanter proposes the addition of effective amounts of dimethyl ether to the process to enhance the separation of the condensed overhead stream in the decanter, e.g., U.S. Pat. No. 7,208,624. However, dimethyl ether is difficult to handle, and the use of dimethyl ether gives rise to controllability problems, especially under steady state conditions, due to low boiling point of dimethyl ether (about 24° C.).
Accordingly, there continues to be a need to further improve the manufacture of acetic acid under low water-high acid conditions. In particular, there continues to be a need to improve and stabilize the phase separation in the decanter to ensure continuous and reliable removal of impurities.